1. Field of the Invention
This invention relates to induction motor drive arrangements and particularly to such arrangements employing a motor convertor supplied from a controllable D.C. source.
2. Description of the Related Art
Such arrangements are known in which the D.C. source is provided by a three-phase thyristor rectifier bridge, the `supply convertor`, which can be phase controlled to determine the D.C. link current between the supply convertor and the motor convertor. One or more D.C. reactors in the D.C. link provide current inertia to maintain the motor convertor current during switching operations.
In addition, it is known to provide a capacitor bank in parallel with an induction motor to provide external excitation, since an induction motor, unlike a synchronous motor for example, will not generate voltage and provide its own excitation.
While an induction motor can be made to operate with excitation capacitors in this way, there is no direct control of the speed.